Exposed: The Mayfair's Mata Hari

With aristocratic links, sex appeal and a skill for extracting secrets from military men, she was Britain's version of Mata Hari.

Today the mysterious Lady Howard of Effingham is exposed in a thousand-page file released at the National Archives.

The documents reveal how the 'highly sexed' aristocrat used her powers of seduction to charm war secrets from RAF officers, politicians and diplomats to pass on to the Germans during the Second World War. Like the exotic dancer Mata Hari - shot for espionage during the First World War - Lady Howard was attractive and skilled at wheedling sensitive information from her high-society contacts.

According to one MI5 report, having procured secret dossiers, she would leaf through them while having her hair styled at Antoine's, a Mayfair hairdresser on Dover Street. In 1941 Lady Howard, then 33, was interned at Holloway women's prison for 'working with the enemy'.

But only now have details of her activities been made public. Posing with an air of 'naive vagueness', she would gather secrets from unsuspecting victims at London cocktail parties, the file claims. During the war, she took a day job at the All Service Canteen Club, where she flirted with military officers.

By night, she developed 'very friendly' contacts with the ambassadors of Russia, Turkey and Egypt. She even made friends with the soon-to-be Foreign Secretary Anthony Eden and his wife — he was warned off her by MI5. The file describes her intimate friendship with a Major Mitchell of the War Office, to whom she once gave £15. There were many others of interest to 'Manci', as she was known.

'One in particular, a young man named Edmondson, aged about 20, is obviously extremely struck with Manci's charms, and appeared to be talking a little freely on technical matters in front of Manci,' an intelligence officer warned in September 1939.

Lady Howard was a Polish refugee named Malwina Gertler who arrived in Britain in 1935, at 27. She soon married 33-year-old Lord Howard of Effingham, purely to gain British nationality, the file claims.

Her true love was the notorious gun-runner Edward Weisblatt, suspected of working for the Gestapo and the Soviet secret police. She mostly lived at Weisblatt's apartments at the Dorchester Hotel and toured London in his chauffeur-driven Rolls Royce.

The security services believed she divulged military secrets to Weisblatt, who passed them on to the Germans via the Russians as part of his business dealings.

Lady Howard had first come to the attention of the authorities in 1936, following a complaint from a man that his wife had fallen under her influence and that she was 'leading an immoral life'.

'A not unattractive gypsy gamin type'

Another acquaintance, Lord Cottenham, told MI5 how he met her over 'a few casual dry martinis' at a party in 1939. 'A not unattractive gypsy gamin type; highly sexed, I should say; possesses considerable facility for copying the Mayfair chatter with an accent more foreign than it need be; but does not always simultaneously smile with her eyes, which are those of a thoroughly intelligent monkey,' he reported.

Weisblatt, meanwhile, came to the attention of the French secret service which reported he was in contact with the German Gestapo and the Russian GPU.

'Under a pose of naive vagueness, she hides an extremely astute mind. She appears to be very attractive to a certain type of man.'

His concerns were echoed by Sir Robert Vansittart, a senior official at the Foreign Office.

He asked 'that this alien undesirable should be interned as soon as possible'.

Lady Howard was released after five months in prison, complaining she had no idea why she had been jailed. Her fate since is unknown.

The files also reveal how a lovestruck English teenager persuaded a Soviet spy to defect to Britain. Isabel Streater, 19, won the heart of Georges Agabekov, 35, after a romance in Istanbul where they met in 1930.

When Miss Streater, the daughter of a businessman in the City, left Istanbul for Paris, her suitor followed, revealing he had intercepted and decoded messages sent from the British High Commissioner in Egypt to the Foreign Office.

She persuaded him to turn his back on Moscow by promising to marry him if he gave up espionage, the file says.